/Engine/Source/Runtime/CoreUObject/Public/AssetRegistry/CookTagList.h
Tags that start with "Diff_" are used for bulk data diff blaming - see DiffAssetBulkDataCommandlet.cpp. When adding diff tags, be sure to add explanatory prose in that commandlet to GBuiltinDiffTagHelp.
void Add ( [UObject](API\Runtime\CoreUObject\UObject\UObject) * InObject, [FName](API\Runtime\Core\UObject\FName) InTagName, [FString](API\Runtime\Core\Containers\FString) && InTagValue )